Method for avoiding corrosion

ABSTRACT

The invention relates to a method of avoiding corrosion in the separation of methylamine from a product stream ( 10 ) which is obtained in the preparation of methylamines by gas-phase reaction of methanol and ammonia and comprises monomethylamine, dimethylamine, trimethylamine, ammonia and methanol as components, where ammonia is separated off by pure distillation in a first column ( 1 ), the remaining components of the product stream obtained as bottoms ( 12 ) are fed to a second column ( 2 ). Trimethylamine ( 14 ) is separated off in the second column ( 2 ) by extractive distillation with introduction of water. The further components of the product stream obtained as bottoms ( 15 ) from the second column ( 2 ) are fed to a third column ( 3 ), in which monomethylamine and dimethylamine are separated off at the top. The monomethylamine and dimethylamine are separated by distillation in a fourth column ( 4 ). To avoid corrosion alkali metal hydroxide is added to the second or third column ( 3 ).

The present invention relates to a method of avoiding corrosion in theseparation of methylamines from a product stream obtained in thepreparation of methylamines.

The reaction of methanol and ammonia in the presence of a heterogeneouscatalyst based on aluminum oxide forms monomethylamine, dimethylamineand trimethylamine. The reaction to form the methylamines is exothermicand occurs at from 390 to 430° C. Since the reactions to formmethylamines are equilibrium reactions, not only methylamines but alsoammonia and methanol are present in the product stream.

After the reaction, the product gas stream is passed to a distillationplant. In the distillation plant, the product gas stream is separatedinto the individual components. In the distillation plants used atpresent, ammonia is separated off in a first column, the ammonia formingan azeotrope with trimethylamine, whereby also a part of thetrimethylamine is distilled off, trimethylamine is separated off in asecond column and water, which generally contains methanol which has notbeen reacted in the reaction, is separated off in a third column. A gasstream comprising monomethylamine and dimethylamine is taken off at thetop of the third column and is passed to a fourth column. In the fourthcolumn, the gas stream is separated into monomethylamine anddimethylamine. To separate off the methanol from the methanol-containingwater from the third column, a further column can be installeddownstream of the third column. The methanol obtained in the furthercolumn is, like the ammonia separated off in the first column, fed backinto the methylamine synthesis.

Owing to the aggressive nature of the components in the product stream,the distillation columns which are preferably manufactured of carbonsteel corrode. The addition of alkali metal hydroxide to the feed to thefirst column in alkylamine plants in order to prevent corrosion isknown. However, when alkali metal hydroxide is added to the feed to thefirst column of the methylamine plant, blockages occur on the trays ofthe first column after only a short time.

It is an object of the present invention to provide a method of avoidingcorrosion in the columns of the distillation plant in the preparation ofmethylamine.

We have found that this object is achieved by adding alkali metalhydroxide to the feed to the third column. When the alkali metalhydroxide is added to the feed to the third column, it is found that nocorrosion occurs in the first and second column, although aggressivemedia are present in the gas stream there. Furthermore, the problemsolution provided by the present invention avoids the formation ofblockages in the first and second columns.

To isolate monomethylamine, dimethylamine and trimethylamine from theproduct gas stream obtained from the reaction of ammonia and methanol,the product gas stream is fed to a distillation plant. The product gasstream is fed into a first distillation column at a side inlet. In thefirst column, ammonia is separated off by pure distillation. Thedistillation is carried out at a pressure of preferably from 15 to 20bar, in particular at a pressure of from 15 to 18 bar. The ammoniaseparated off as azeotrope with trimethylamine is taken off at the topof the first column and is preferably recirculated to the preparation ofmethylamine. The other constituents of the product gas stream form thebottoms and are taken off from the column and fed to a second column.The feed to the second column is likewise introduced at a side inlet. Inthe second column, trimethylamine is separated off by extractivedistillation with addition of water. The trimethylamine is taken off atthe top of the second column. The remaining components of the productgas stream which form the bottoms are fed into a third column at a sideinlet. The water used for the extractive distillation in the secondcolumn and the water formed in the reaction and also unreacted methanolare taken off at the bottom of the third column. At the top of the thirdcolumn, a mixture of monomethylamine and dimethylamine is taken off. Thedistillation in the third column is preferably carried out at a pressureof from 7 to 15 bar, in particular at a pressure of from 8 to 12 bar.The mixture of monomethylamine and dimethylamine taken off at the top ofthe third column is fed into a fourth column at a side inlet. In thefourth column, the stream comprising monomethylamine and dimethylamineis fractionally distilled at a pressure of preferably from 6 to 10 bar,in particular at a pressure of from 7 to 9 bar. Dimethylamine isobtained at the bottom of the fourth column and monomethylamine is takenoff at the top of the fourth column.

To separate off the methanol from the water obtained in the distillationin the third column, it is possible to use a fifth column into which themethanol-containing water from a side offtake of the third column isfed. In the fifth column, the methanol is separated off by distillation.The methanol is taken off at the top of the fifth column and isrecirculated to the reaction. The bottoms from the fifth column consistof water which has been freed of methanol and are recirculated to thethird column.

In the problem solution provided by the present invention for avoidingcorrosion in the distillation plant, alkali metal hydroxide is added tothe feed to the third column. Alkali metal hydroxides which are suitablefor the purposes of the invention are, in particular, sodium hydroxideand potassium hydroxide. The amount of alkali metal hydroxide has to besufficient for unreacted alkali metal hydroxide to be present in thebottoms from the third column.

The columns are preferably heated by means of steam at a pressure ofpreferably from 10 to 20 bar and in particular a pressure in the rangefrom 12 to 17 bar at the bottom of the columns.

The columns used for the distillation are preferably tray columns.Column trays of all construction types known to those skilled in the artare suitable. Apart from tray columns, it is also possible to use packedcolumns. Here, any packing geometry known to those skilled in the artcan be used.

The product stream is preferably transported through the cascade ofcolumns by means of the pressure difference between the individualcolumns.

The product gas stream comprises not only monomethylamine, dimethylamineand trimethylamine but also methanol and ammonia which have not reactedin the reaction and water formed as reaction by-product together withfurther by-products. Among these by-products, carbon monoxide, carbondioxide, ammonium carbamate and formic acid in particular have acorrosive action toward iron. Addition of a base to neutralize the acidsand to produce a basic environment can reduce or prevent corrosion ofthe iron.

As base to avoid corrosion, preference is given to adding alkali metalhydroxide, in particular sodium hydroxide or potassium hydroxide. Thealkali metal hydroxide can be added in solid form or preferably as anaqueous solution. When an aqueous alkali metal hydroxide solution isused, this preferably has a concentration of 25%.

It has surprisingly been found that when sodium hydroxide is added tothe feed to the third column, no corrosion occurs in the distillationplant and no trays become blocked. Although the composition of theproduct stream in the second column differs from the composition in thethird column only in that trimethylamine is present in the productstream in the second column and additional water is added in the thirdcolumn, no corrosion occurs in the second column when the alkali metalhydroxide is added to the feed to the third column.

The amount of alkali metal hydroxide introduced has to be such thatalkali metal hydroxide is still present in the bottoms from the thirdcolumn.

Apart from the addition to the feed to the third column, the alkalimetal hydroxide can also be added to the bottoms from the second columnor be introduced directly into the stripping section of the secondcolumn.

The invention is illustrated below with the aid of a drawing and anexample.

FIG. 1 shows a distillation plant configured according to the presentinvention for methylamine distillation.

A distillation plant configured according to the present invention formethylamine distillation comprises five columns as shown in FIG. 1. Aproduct gas stream 10 obtained in the reaction of ammonia and methanolto form methylamines is fed into a first column 1 via a side inlet. Inthe first column 1, ammonia is separated off as an azeotrope withtrimethylamine from the product stream by distillation. Ammonia 11 istaken off at the top of the first column 1 and is recirculated to themethylamine synthesis. The remaining components of the product stream 10are obtained as bottoms 12 from the first column 1. The bottoms 12 fromthe first column 1 are fed into a second column 2 via a side inlet. Inthe second column 2, trimethylamine is separated off from the bottoms 12from the first column 1 by extractive distillation. For the extractivedistillation, water 13 is introduced into the second column 2 via asecond side inlet. The second side inlet is located above the inlet forthe bottoms 12 from the first column 1. Trimethylamine 14 is taken offat the top of the second column 2. The remaining components collect inthe bottoms 15 from the second column 2. The bottoms 15 from the secondcolumn 2 are fed as feed 17 to a third column 3. Alkali 16 is added tothe feed 17. Apart from the addition of the alkali 16 to the feed 17 tothe third column 3, the alkali 16 can also be introduced into thebottoms 15 or into the stripping section of the second column 2. Thealkali 16 is preferably an alkali metal hydroxide, in particular sodiumhydroxide or potassium hydroxide in aqueous solution.

In the third column 3, monomethylamine and dimethylamine are separatedoff from the bottoms 15 from the second column 2 by distillation. Themonomethylamine and dimethylamine are taken off as overhead stream 18from the top of the third column 3 and fed to a fourth column 4. Water,methanol and further reaction by-products are present in the bottomsfrom the third column 3. To separate off the methanol from the bottomsfrom the third column 3, a fifth column 5 can be installed downstream ofthe third column 3. Methanol-containing water from the third column 3 isfed into the fifth column 5 via an inlet 20. In the fifth column 5,methanol is separated off from the water by distillation. The waterwhich has been freed of methanol is recirculated via a return line 21 tothe third column 3. The methanol 22 separated off is taken off at thetop of the fifth column 5 and is recirculated to the methylaminesynthesis. At the bottom of the third column 3, wastewater 19 which hasbeen freed of methanol is taken off.

In the fourth column 4, the overhead stream 18 from the third column 3,which comprises, in particular, monomethylamine and dimethylamine, isseparated into monomethylamine and dimethylamine. Monomethylamine 24 istaken off at the top of the fourth column 4. The dimethylamine 23obtained as liquid phase in the fourth column 4 is taken off at thebottom of the fourth column 4.

EXAMPLE

A product stream obtained in the synthesis of methylamine isfractionated in a distillation plant. The columns are heated by means ofsteam at a pressure of 16 bar. In the first column, ammonia is separatedoff as an azeotrope with thrimethylamine from the product stream at apressure of 16.5 bar. The remaining product stream is fed to a secondcolumn. In the second column, trimethylamine is separated off at apressure of 14 bar and a temperature at the bottom of 160° C. and atemperature at the top of 103° C. and is taken off at the top of thesecond column. The bottoms from the second column are fed to a thirdcolumn, with sodium hydroxide solution being added to the feed stream.In the third column, dimethylamine and monomethylamine are taken off atthe top at a pressure of 8.3 bar and a temperature at the bottom of 178°C. and a temperature at the top of 68° C. The monomethylamine anddimethylamine are separated in a fourth column at a pressure of 7.5 barand a temperature at the bottom of 74° C. and a temperature at the topof 53° C. Monomethylamine is taken off at the top of the fourth columnand dimethylamine is taken off at the bottom of the fourth column. Themethanol-containing water obtained via a side take-off in bottom of thethird column is fed to a fifth column in vapour state which is operatedat a pressure of 8.3 bar and a temperature at the bottom of 170° C. anda temperature at the top of 165° C. Methanol is taken off at the top ofthe fifth column. The water which has been freed of methanol is takenfrom the bottom of the fifth column and is fed back into the thirdcolumn. The sodium hydroxide solution used has a concentration of 25% ofNaOH. No corrosion in the columns is observed in the methylaminedistillation carried out in this way.

List of Reference Numerals

-   1 First column-   2 Second column-   3 Third column-   4 Fourth column-   5 Fifth column-   10 Product stream-   11 Ammonia-   12 Bottoms from the first column 1-   13 Water-   14 Trimethylamine-   15 Bottoms from the second column 2-   16 Alkali-   17 Feed to the third column 3-   18 Overhead stream from the third column 3-   19 Wastewater-   20 Feed to the fifth column 5-   21 Recycle stream to the fifth column 5-   22 Methanol-   23 Dimethylamine-   24 Monomethylamine

1. A method of avoiding corrosion in the separation of methylamines froma product stream (10) which is obtained in the preparation ofmethylamines by gas-phase reaction of methanol and ammonia and comprisesmonomethylamine, dimethylamine, trimethylamine, ammonia and methanol ascomponents, where ammonia (11) is separated off by pure distillation ina first column (1), the remaining components of the product streamobtained as bottoms (12) are fed to a second column (2), trimethylamine(14) is separated off in the second column (2) by extractivedistillation with introduction of water, the further components of theproduct stream obtained as bottoms (15) from the second column (2) arefed to a third column (3), monomethylamine and dimethylamine areseparated off in the third column (3) and the monomethylamine anddimethylamine are separated by distillation in a fourth column (4),wherein alkali metal hydroxide is added to the second or third column(3).
 2. A method as claimed in claim 1, wherein an additional fifthcolumn (5) into which a stream taken from a side offtake or the bottomof the third column (3) is fed and in which methanol is separated off bydistillation is installed downstream of the third column (3).
 3. Amethod as claimed in claim 2, wherein the methanol-free water obtainedas bottoms from the fifth column (5) is recirculated to the third column(3).
 4. A method as claimed in claim 1, wherein the alkali metalhydroxide is added to the feed (17) to the third column (3).
 5. A methodas claimed in claim 1, wherein the alkali metal hydroxide is added tothe bottom (15) from the second column (2) or is introduced into thestripping section of the second column (2).
 6. A method as claimed inclaim 1, wherein the amount of alkali metal hydroxide added is such thatalkali metal hydroxide is still present in the bottoms from the thirdcolumn (3).
 7. A method as claimed in claim 1, wherein the alkali metalhydroxide is sodium hydroxide.
 8. A method as claimed in claim 1,wherein the alkali metal hydroxide is potassium hydroxide.